Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
Printed images generated using ink jet technology are subject to environmental degradation. They are especially vulnerable to damage resulting from contact with aqueous solutions and atmospheric gases such as ozone. The damage resulting from post-imaging contact with aqueous solutions can take the form of water spots resulting from deglossing of the top coat, dye smearing due to unwanted dye diffusion, and even gross dissolution of the image-recording layer. Ozone destroys ink jet dyes resulting in loss of density.
Several methods for minimizing the effects of environmental degradation are well known in the art of ink jet printing. Printed images may be laminated with a clear overcoat film. However, lamination is expensive as it requires a separate roll of material, and it is often time consuming and difficult to evenly apply the film to the printed image without any creases or pockets of trapped air.
Another method for minimizing the effects of environmental degradation requires the use of an ink jet recording element that is fused using heat and/or pressure after the image has been printed. These fusible inkjet recording elements typically have a construction of at least two layers coated on a support wherein the top layer is porous and consists mainly of polymeric particles. Upon printing, the ink is transported through the top layer and into at least one layer underneath. The printed image is then fused and the polymeric particles in the porous top layer soften and form a continuous nonporous layer that resists water and staining. However, this method is expensive because it requires specially designed ink jet recording elements and an integral or peripheral fusing station.
Another method for minimizing the effects of environmental degradation is to apply a coating of a polymeric solution or dispersion on the surface of the printed image using a brush, roller, sponge, etc. A protective overcoat forms as the solution or dispersion dries. However, this method is considered too impractical and undesirable for use by the typical consumer.
Another method for protecting printed images from environmental degradation is the application of a polymeric solution or dispersion to the printed image using an ink jet printer. This method has been described for use in ink jet printers used in desktop and wide-format graphic applications and mostly for printers which utilize aqueous-based inks. The polymeric solutions, also known as overcoat solutions, are formulated with polymer, water, and other components commonly used in aqueous-based ink jet ink formulations, for example, humectants, organic solvents, surfactants and biocides. The overcoat solution is loaded into an ink jet printhead, positioned in the carriage assembly of the printer and then instructed to jet the overcoat solution over the printed image.
U.S. Pat. App. 2002/0009547 A1 describes a coating liquid and image recording method that provides a transparent topcoat for recordings. The topcoat is formed by jetting an aqueous solution of fine polymer particles onto a recorded image. The fine polymer particles are present in the form of an aqueous emulsion made up of a resin or resins having an acid value of 100 or less and being insoluble in water. The problem with this coating liquid is that the resulting topcoat is not as resistant to common household stains as one would like, as will be shown hereafter.
U.S. Pat. No. 5,640,187 describes an ink jet recording method having a step in which a solution containing a resin having an acid value of 30 to 100 and soluble in alkali is discharged to a recording member prior to or after the ink is discharged. However, Applicants have found that a film formed from such a resin is not as resistant to common household stains as one would like, as will be shown hereafter.
U.S. Pat. App. 2002/0065339 A1; G.B. Pat. App. 2,337,482 A and European Pat. App. 1,186,439 A1 describe compositions that are used to overcoat ink jet recorded images using an ink jet printer. While these applications describe the use of acidic polymers, they do not teach that the acid number of a polymer can be used to predict its performance as an overcoat.
It is an object of the present invention to provide an overcoat composition that can be applied over a printed image. It is another object of the present invention to provide an overcoat composition that forms a protective film on a printed image and is highly resistant to staining. Yet another object of the present invention is to provide a protected printed image that is highly resistant to a variety of aqueous-based solutions. Such an overcoat composition is applicable to various image recording elements, including, for example, ink jet, photographic, electrophotographic, thermal dye transfer, thermographic and lithographic.